1. Field of the Invention
This invention relates to thermal reduction of calcium sulfate to produce sulfur dioxide and a useful solid byproduct and, more particularly, to the reduction of calcined phosphogypsum using a fossil fuel to make sulfuric acid and slag for use as aggregate.
2. Discussion of Prior Art
Phosphogypsum is an unwanted byproduct of the manufacture of phosphoric acid for fertilizer using a wet process. For many years, fertilizer producers have been seeking a means to recover sulfur from this byproduct to make sulfuric acid to be recycled into the wet process. Steady accumulation of phosphogypsum in huge "gyp stacks" can thereby be avoided and the existing accumulation reduced. Furthermore, by recovering sulfur from the byproduct, the high cost of sulfur used to make sulfuric acid can be avoided.
Numerous processes have been developed over the years to free the sulfur trapped within calcium sulfate in the phosphogypsum. The prior art processes are flawed in that they either do not recover sufficient SO.sub.2 from the phosphogypsum, are energy inefficient, or do not yield a useable solid end product. One example of a prior art process is French Patent 1,437,484, which discloses using electrolysis for reduction of the phosphogypsum. Bacterial action and intermediate-temperature thermal reduction to calcium sulfide, followed by reaction with water and carbon dioxide is disclosed in Starnes, E. M., Chairman Florida Institute of Phosphate Research Priorities (August 1985). South African Patent 68/5571 to Dorr-Oliver discloses using carbon monoxide and hydrogen as reducing gases. U.S. Pat. Nos. 3,087,790; 3,260,035; 3,607,045 and 4,102,989 to Wheelock describe a process for the high-temperature thermal reduction of phosphogypsum to quick lime, and high-temperature thermal reduction of phosphogypsum to lime-related products such as Portland cement.
Various processes for the high-temperature thermal reduction of phosphogypsum to lime or limerelated products have received the greatest development effort. U.S. Pat. No. 4,774,064 to Arnold et al. teaches combusting fuel with gypsum dispersed in a gas stream, thereby subjecting the calcium sulfate mineral to temperatures in excess of 3400.degree. F. The particles are allowed to fall through this downward-flowing gas in a drop tube reactor. All of the fuel is burned to provide process heat. Arnold et al. prevents slag deposition on the walls of the reactor by maintaining a boundary layer of inert gas adjacent thereto. Arnold et al. views the prevention of slag buildup as an advantage of his process and an improvement over the prior art.
Without a slag layer to bring different particle types into contact, extend their retention time and thus help complete the requisite chemical reactions, extreme temperatures are required by Arnold et al. for adequate conversion. The flame zone of Arnold et al. is at 2440.degree. C. Consequently, fuel requirements are high. Significant amounts of costly oxygen, without which such high reaction temperatures probably cannot be reached, are also required by this process.
Other prior art high-temperature thermal processes for the conversion of calcium sulfate to cement clinker include the Mueller Kuhne cement process, which was demonstrated in Germany during World War I, when importation of Spanish pyrites was cut off. This rotary kiln process produced cement clinker as the solid, calcium-containing byproduct, in addition to sulfur dioxide for making sulfuric acid. That technology was the foundation for what is now known as the OSW-Krupp process, practiced commercially in South Africa and in Austria. Related plants are in use in Poland and in East Germany. These are the only commercial units known to be still in operation for production of sulfur dioxide from the various forms of calcium sulfate. There are none in the United States. The disadvantages of the rotary kiln process are residence times in excess of six hours at temperatures of typically 2700.degree. F., thereby consuming about 21 million BTU's per ton of cement clinker produced.
Besides the rotary kiln, various other reactor configurations have been developed, or are being tried experimentally, for high-temperature thermal reduction. These include the two fluidized bed stages disclosed in South African Patent 68/5571, which is designed to avoid prevent the build-up of slag and to produce solid calcium sulfide (CaS) by strong reduction in conventional bubbling fluidized beds.